Collaborators: Steve Pennings

The factors that control plant abundance and diversity in natural systems have been well-studied, and many of these factors (climate, land management and macro- and micro-nutrient cycling) are changing due to anthropogenic influences.  Despite decades of interest, less is known about the relative importance of factors controlling the abundance and diversity of herbivores.  The complexity of herbivore habitat and food requirements makes it difficult to develop simple models that adequately explain variation in herbivore communities.  Different studies have variously ascribed control of herbivore community structure to a wide range of factors, including plant biomass, diversity, nitrogen content, protein content and the concentration of various micronutrients, and to top-down control from natural enemies. Few studies, however, have examined all these potential factors or their drivers simultaneously, and little is known about their relative importance in most natural systems.

From first principles, the simplest hypotheses one could make about herbivore communities are that herbivore density is mediated by plant abundance (and perhaps nitrogen content), and that herbivore diversity is mediated by plant diversity.  These hypotheses suggest that plant micronutrients play a minor role in structuring herbivore communities.  Preliminary data from a grassland study site, however, suggest that plant micronutrients can be very important in mediating herbivore community structure.  These data show that plant community attributes alone do not adequately predict the density or richness of herbivores, but a major impact to soil micronutrients was a strong predictor of herbivore density or richness.  These data suggest that a multi-factorial approach to understanding herbivore community structure is likely to be more insightful than exploring any single driver in isolation.

Understanding what structures herbivore communities is important from both a theoretical and practical perspective.  Herbivores play important trophic roles by providing top-down control on plant communities and as food for predators.  Insect herbivores are important in grassland management, first because they provide food for threatened birds and may compete with grazing vertebrates, and second, because grassland management strategies (mowing, burning, grazing) feed back to affect insect herbivore communities through multiple pathways.  This study will take a multi-factorial approach to understanding controls on herbivore community structure in a threatened grassland ecosystem (coastal tallgrass prairie).

We have currently submitted several grant proposals to study these ideas at a coastal tallgrass prairie site owned by the University of Houston, the University of Houston Coastal Center.